The present invention relates generally to a balun in a magnetic resonance imaging (MRI) system, and, more specifically, to a balun shield that includes an integrated capacitor therein.
MRI uses radio frequency pulses and magnetic field gradients applied to a subject in a strong homogenous magnetic field to produce viewable images. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it, in random order, at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR receiver coils receive the emitted electromagnetic signals emanating from the patient and use the acquired signals for image reconstruction. Before image reconstruction occurs, the electromagnetic signals received by the receiver coil elements are conditioned by a balun. The balun helps to improve performance of the receiver coils by providing a high impedance path for common-mode currents in the electromagnetic signal and a low impedance path for differential currents therein. Typically, the balun includes a non-magnetic co-axial conductor wound into a single layer cylindrical coil. The cylindrical coil is then enclosed in a conductive shield and the shield is connected to an output of the conductor by way of a capacitor unit. The capacitor unit, together with the coil, creates a parallel resonant circuit that can be tuned to the imaging frequency to significantly reduce common mode currents propogating down the co-axial conductor.
With this typical design, however, problems can arise with the performance of the balun shield. The balun shield's ability to properly shield the balun (and any other electrical components contained within the balun shield) can be degraded if gaps are present in the balun shield. That is, the connection between the capacitor unit and the coil conductor often requires that a small gap exist in the balun shield to allow for connection of the capacitor unit. As stated above, such a gap can cause instability in components within the balun shield by allowing unwanted feedback of the signal into the interior of the balun.
Therefore, a balun shield design that reduces or eliminates such gaps is greatly desired to improve overall performance.